There are primarily three types of physical media utilized in wired communication systems: (1) fiber optic cable, (2) coaxial cable, and (3) twisted copper pair cable ("twisted pair"). Presently, twisted pair cable predominates in the access local loop portion of existing telephone networks. Coaxial cable has been used widely by cable television companies, and both telephone companies and cable companies have made use of fiber optics for main or trunk line signal distribution.
Fiber optic cable can carry more information over greater distances than coaxial cable, while coaxial cable can carry more information over greater distances than twisted pairs. Because twisted pair is the predominant local loop technology in the telephone industry, new network architectures have been developed that work with twisted pair wire to increase information carrying capacity. In the cable industry, an HFC network employs a combination of broadband linear fiber optics and coaxial cable. Such a network also allows delivery of many advanced two-way services in a cost-effective manner when compared with total conversion to a baseband digital optical network with significant time-division multiplex hardware included in the access plant.
In addition to requiring two-way communication, voice telephony service has two other requirements not necessarily addressed by video distribution networks: lifeline service powering and privacy of voice communication. In video networks, the power to operate a subscriber television set for example, is provided by the subscriber, i.e., a subscriber merely plugs a television into an electrical outlet at the subscriber location. In the event of a power outage, the user is unable to view the delivered video services.
In contrast, telephony subscribers expect working telephone service whether or not electricity is available. Because basic telephone service is always expected to be available, the telephone distribution network typically provides to a subscriber location what is referred to as "talk battery" along with electric power utilized for conventional voice telephony signaling and supervision to ensure the availability of such "lifeline" services.
Basic HFC two-way capability requires the addition of reverse-path node laser transmitters and headend receivers, and activation of two-way amplifiers in the coaxial portion of the network. Node power and all locally powered amplifiers require appropriate uninterruptible power supplies (UPS) in order to bring basic HFC system reliability to the proper level for support of lifeline telephony service availability.
Conventional tap devices do not pass power to the drop cable. In some HFC networks, reverse-path filters are used to mitigate noise ingress in the reverse spectrum, and the maximum value of through current may only be 4 to 6 amperes at 48 volts AC. Such through current and relatively low voltage is inadequate to provide the necessary reserve power for cable telephony remote hardware in HFC networks being upgraded for lifeline telephony services.
Providing power for "talk battery" and emulation of conventional telephony signaling and supervision on HFC networks is usually accomplished in one of two ways, either as steady-state power supplied only from the network as shown in FIG. 1, or as Exclusive-OR local rectifier/battery arrangements at or near a customer residence as shown in FIG. 2.
More specifically, the hybrid fiber/coaxial (HFC) cable network of FIG. 1 includes a cable headend/central office 10 incorporating a central office switch 12 connected to a two-way digital interface or head-end digital terminal (HDT) 14. HDT 14 modulates the digital signals from the line side of switch 12 to generate appropriate modulated RF carriers 16 for transmission on a fiber optic cable 18 by a combiner/optical transceiver 20, and to demodulate RF carriers received by optical transceiver 20 from cable 18.
Central office 10 further includes a bank of RF modulators 22 for generating modulated video/audio signals from baseband signals received from an external baseband video service 24. Optical transceiver 20 is arranged to transmit the modulated video signals onto cable 18. Baseband special services equipment 26 is also connected to the network via HDT 14 and transceiver 20.
An optical node 28 includes a suitable optical transceiver 30 coupled to fiber optic cable 18 and a coaxial cable 32 for providing a link for communications between the central office 10 and television units 34 and cable telephony service platform (CTSP) equipment 36 connected to telephone units 37 located at one or more customer premises 38. In the network power only arrangement, electrical power is generated by a protected power supply 40 located at a node, and powers amplifiers 42, and all equipment in a network interface unit (NIU) 44 located at or near a customer's premises via a power passing tap 46 and a drop cable 47. Power supply 40 also provides power for lifeline "talk battery" along with electric power utilized for conventional voice telephony signaling and supervision.
FIG. 2 illustrates the typical local power arrangement for an HFC network, wherein elements identical to those described in connection with FIG. 1 have been denoted with like reference numbers. As shown, a conventional tap 49, i.e., a nonpower passing tap, is employed in the network so that NIU 44 does not receive equipment power from node 28. Instead, electric power for talk battery and conventional voice telephony signaling and supervision is provided by an AC power converter/battery charger unit 48 connected to a conventional AC power source 50 and a back-up battery 52. In the event of power loss from AC power source 50, CTSP 36 within NIU 44 will receive power from battery 52.
Regarding the network-only power arrangement, safety regulations in certain countries limit voltage on a coaxial cable plant to a typical value of 48 vAC, and certain active and passive devices pass only about one-third of the current required to support cable telephony lifeline service applications. Thus, network power source arrangements have not proven satisfactory in certain situations.
With respect to the local power arrangement, reliability is dependent on considerations external to the network like AC power source reliability or user interaction to maintain properly charged batteries. In addition, a common practice in certain countries is to turn off all circuit breakers and unplug all appliances prior to departing on month-long holidays. Therefore, inadvertent or deliberate disconnection of ordinary lead-acid batteries servicing cable telephony network interface units (NIU) on customer premises are deprived of a constant float voltage for extended periods, which in turn leads to likely battery damage or destruction well before the rated operational life expectancy of the battery.
As such, a need exists for an apparatus and method of reliably enabling the electrical form of talk battery, signaling, and supervision in an HFC network which can support telephony lifeline service applications in a cost effective manner while also minimizing the need for customer interaction at the customer location, and power drawn from network optical node or other network uninterruptible power supplies.